This invention relates to communications systems, microwave communications systems, and specifically an electro optic microwave communications system.
As the information age has grown, the availability of wireless bandwidth has been seriously constrained by available spectrum. The obvious growth path, as spectrum becomes fully occupied or oversubscribed, is to move upward in frequency to unused bandwidth. The upper frequency limit has steadily risen to where the use of the millimeter and terahertz (THz) spectrum is now realistic. Building radio frequency (RF) circuitry for these frequency ranges is very difficult and expensive and in particular a unique system must be built for each sub-band. No known technology exists for generation (transmit) and reception (receive) of signals across multiple sub-bands or indeed across the entire band (up to 1 THz). Various absorption holes exist in the THz frequency range and hence useful communications is possible in these holes. Government surveillance systems are already vitally concerned with signal activity in this range and are hampered by the lack of RF technology to prosecute such signals.
The state-of-the-art today finds commercial communications up to 67 gigahertz (GHz), and military uses to 94 GHz and beyond. Much experimentation is underway in the terahertz range, and various specialized users are already demanding receive capability in the 100- to 300-GHz range. From a pure atmospheric attenuation perspective shown in FIG. 1, it can be seen that there are several low-attenuation bands at 140, 240 and 560 GHz usable for free space communications. Many uses, however, do not rely on low-attenuation bands, and can utilize any terahertz frequency for deliberate limited range communication.
Current technology is slowly evolving toward an expanded set of block converters, each using a unique technology optimized to a particular frequency band to cover the terahertz frequency range. Typically, the block converter includes an antenna, preamplifier and mixer driven by a fixed, multiplied local oscillator. Two or more block converters may be used in cascade.
What is lacking is the technology to implement a continuous-tuning communications or surveillance system that covers the 20–500 GHz band, similar to the common availability today of receivers that cover 20–520 megahertz (MHz).